One diet myth is that certain foods metabolize fat simply because they're so difficult to digest. Operating on the theory that a certain number of calories are needed to chew and process high-fiber, complex carbohydrate foods, proponents of "negative calorie" foods assert that you metabolize excess fat simply by consuming them. However, Dr. Donald Hensrud, the Mayo Clinic, says there's no proof that certain foods create a calorie deficit.
The Concept
Calories are the energy units you get from food -- the energy that fuels your activities of daily living, be it simply walking across the room or engaging in strenuous physical activity. All foods have calories in varying amounts. However, the premise behind the negative calorie diet is that some foods, particularly complex carbohydrates that are high in insoluble fiber, take more energy to burn than they do to eat and can therefore result in weight loss. According to EveryDiet.org, a diet high in these types of food isn't necessarily unhealthy -- complex carbohydrates give you the fuel you need for energy, and fiber encourages good digestion and regular bowel movements.
The Foods
High on the list of so-called fat-burning foods are green vegetables such as broccoli and celery, which are very low in calories and high in hard-to-digest fiber. Hensrud also says other foods that rank as "fat metabolizers" include cabbage, grapefruit, zucchini, asparagus, cauliflower and berries. He adds that proponents of the negative calorie diet theory claim you'll burn 80 calories by eating a piece of broccoli, which has 25 calories, creating a 55-calorie deficit.
The Reality
Your body expends roughly 10 percent of its daily energy processing food and storing nutrients, Hensrud says. Thermogenesis is a relatively minor factor in determining how many calories you burn each day. The more pressing factor is your level of physical activity. There have been no studies conducted to determine if consuming "negative calorie" foods burn fat. EveryDiet.org states that if eating "fat-burning" foods causes you to lose weight, this is likely because the foods themselves contribute to a low-calorie diet. However, their fat-metabolizing effect is questionable. In a May 2009 article in "Time" magazine, Nancy Synder, M.D. says, "Although chewing celery might seem like a strenuous activity, it burns about the same amount of calories as watching grass grow."
Creating a Real Calorie Deficit
Heading for the salad bar or reaching for the raw vegetable tray may not create the calorie deficit you anticipate, if you end up consuming more calories than you can burn each day. You can, however, create a true calorie deficit by finding out how many calories you need to eat to obtain your desired weight and reducing the number you consume. American Council on Exercise dietitian Natalie Digate Muth indicates that if you want to lose a pound a week, you need to cut 3,500 calories from your diet. You can do this by eating 500 calories less per day, or by taking a two-mile run and skipping a 300-calorie dessert, she says. Calorie reduction -- not starvation -- and increased physical activity are still the proven methods for certain weight loss.
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